Development of quick neutron spectrum reconstruction module based on POD for burnup calculation of fast reactor

A quick neutron spectrum reconstruction module based on proper orthogonal decomposition (POD) and a regression model for fast reactors is developed for the general-purpose burnup calculation code SWAT-X. This module enables burnup calculations to be performed quickly for arbitrary conditions based on a few parameters, eliminating the need for computationally intensive calculation codes. Reduced-order and regression models are constructed by applying the POD technique to the snapshot data of the multi-group neutron fluxes calculated using randomly-sampled parameter sets for a single hexagonal pin-cell of a sodium-cooled fast reactor. The neutron spectrum is reconstructed using a linear combination of POD basis vectors with the coefficients obtained from the regression model. The applicability of a pin-cell model, the accuracy of the reconstruction module itself, and its integration with SWAT-X are verified by comparing with the calculation results from the reactor physics code system CBZ. The comparison of nuclide inventories between pin-cell and whole core models shows an agreement within 5% for major actinoids and fission products with relatively large inventories. The developed module reproduces the neutron spectrum above several hundred eV with an accuracy of a few percent using up to six POD basis vectors. It also predicts the 1-group cross sections with an accuracy of a few percent for all parameter sets when the reduced-order model is used with eight POD basis vectors and third-degree polynomial regression. Furthermore, SWAT-X implementing the developed module reproduces major actinoid compositions within approximately 6% of the CBZ results.